1. Field of the Invention
The present invention relates to a grain-oriented electrical steel sheet for use in the cores of transformers and the like and to a method of producing the same.
2. Description of the Related Art
Grain-oriented electrical steel sheet is used primarily as a core material for transformers and generators. The steel sheet has low core loss achieved by utilizing secondary recrystallization during finish annealing in the production process to confer a texture with high-density {110} less than 001 greater than  orientation (Goss orientation). JIS C 2553 classifies the core loss of grain-oriented electrical steel sheet into different grades based on W17/50 (energy loss under excitation conditions of B8 1.7 T, 5.0 Hz).
Transformer cores are of two types, wound and stacked. In order to realize a compact transformer, a designed flux density of higher than 1.7 T, e.g., around 1.9 T, may be used in either type.
Since the stacked core is fabricated by stacking steel sheets like the floors of a building, the flux density may locally become greater than 1.7 T even though the designed flux density is 1.7 T. At higher than 1.7 T, e.g., at W19/50, the transformer core loss is strongly affected.
Increasing awareness of the need for global environmental protection and energy conservation has recently generated demand for grain-oriented electrical steel sheet with still lower core loss, particularly for a steel sheet with low core loss even in a high-intensity magnetic field of, say, 1.9 T. Over many years, there has been seen little or no innovation or improvement aimed at lowering W17/50 core loss to meet this demand.
In light of the foregoing circumstances, the present invention has as its object to provide a high flux density grain-oriented electrical steel sheet low in core loss at excitation flux densities higher than 1.7 T.
The gist of the present invention for overcoming the foregoing issues is as follows.
The present invention provides a high flux density grain-oriented electrical steel sheet excellent in high magnetic field core loss property containing, in percentage by weight, not greater than 0.005% of C, 2.0-7.0% of Si, not greater than 0.2% of Mn, one or both of S and Se in a total amount of not greater than 0.005%, the balance being of Fe and unavoidable impurities, optionally containing, in percentage by weight, not greater than 0.065% of Al, not greater than 0.005% of N and 0.003-0.3% each of one or more of Sb, Sn, Cu, Mo, Ge, B, Te, As, Cr and Bi, the steel sheet having a grain orientation deviating from an ideal {110} less than 001 greater than  orientation by an average of not greater than 5xc2x0, having an average 180xc2x0 magnetic domain width of not greater than 0.30 mm preferably not greater than 0.26 mm or greater than 0.26 mm and not greater than 0.30 mm, and having an area ratio of magnetic domains of a width greater than 0.4 mm of greater than 3% and not greater than 20%.
The method of producing a high flux density grain-oriented electrical steel sheet excellent in high magnetic field core loss property according the present invention comprises a step of making a steel containing, in percentage by weight, 0.015% to not greater than 0.100% of C, 2.0-7.0% of Si, 0.03-0.2% of Mn, one or both of S and Se in a total amount of 0.005-0.050%, optionally containing, in percentage by weight, not greater than 0.065% of Al, not greater than 0.005% of N and 0.003-0.3% each of one or more of Sb, Sn, Cu, Mo, Ge, B, Te, As, Cr and Bi, the balance being Fe and unavoidable impurities, a step of obtaining a starting material by heating and then hot rolling a slab of the steel into a coiled steel sheet or by directly casting a coiled steel sheet from the molten steel, a step of obtaining a steel sheet of final thickness by hot-rolled coil annealing and strong cold rolling, by preliminary cold rolling, precipitation annealing and strong cold rolling, or by hot rolled coil annealing, preliminary cold rolling, precipitation annealing and heavy cold rolling, and a step of subjecting the steel sheet of a final thickness to decarburization annealing, final finish annealing and final coating, which method further comprises a step of rapid heating to a temperature of 800xc2x0 C. or higher at a heating speed of not less than 100xc2x0 C./s immediately prior to the decarburization annealing and a step of effecting magnetic domain control during or at the end of the production process, thereby providing a steel sheet having a grain orientation deviating from an ideal {110} less than 001 greater than  orientation by an average of not greater than 5xc2x0, having an average 180xc2x0 magnetic domain width of not greater than 0.30 mm, preferably not greater than 0.26 mm or greater than 0.26 mm and not greater than 0.30 mm, and having an area ratio of magnetic domains of a width greater than 0.4 mm of greater than 3% and not greater than 20%.